The present invention relates to wireless communications systems and methods, and more particularly, to systems and methods for providing access in wireless communication systems.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have been long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a conventional terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication between a cell 24 and mobile terminals 22. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system""s service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels. As a practical matter well known to those skilled in the art, radiotelephone communications signals, being modulated waveforms, typically are communicated over predetermined frequency bands in a spectrum of carrier frequencies. In a typical FDMA system, each of these discrete frequency bands serves as a channel over which cellular radiotelephones communicate with a cell, through the base station or satellite serving the cell.
The limitations on the available frequency spectrum present several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system may require more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This challenge is heightened because subscribers may not be uniformly distributed among cells in the system. More channels may be needed for particular cells to handle potentially higher local subscriber densities at any given time. For example, a cell in an urban area might conceivably contain hundreds or thousands of subscribers at any one time, easily exhausting the number of channels available in the cell.
For these reasons, conventional cellular systems employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Frequency reuse involves allocating frequency bands to each cell, with cells employing the same frequencies geographically separated to allow radiotelephones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system having only several hundred allocated frequency bands.
Another technique which can further increase system capacity and spectral efficiency is the use of time division multiple access (TDMA). A TDMA system may be implemented by subdividing the frequency bands employed in conventional FDMA systems into sequential time slots. Communications over a frequency band typically occur on a repetitive TDMA frame structure that includes a plurality of time slots. Examples of systems employing TDMA are those conforming to the IS-136 standard, in which each of a plurality of frequency bands are subdivided into 3 time slots, and systems conforming to the GSM standard, which divides each of a plurality of frequency bands into 8 time slots. In these TDMA systems, each user communicates with the base station using bursts of digital data transmitted during assigned time slots.
A channel in a TDMA system typically includes at least one time slot on at least one frequency band. Typically included among the channels in a TDMA system are dedicated control channels, including forward (downlink) control channels for conveying information from a base station to subscriber terminals, and reverse control channels for conveying information from subscriber terminals to a base station. The information broadcast on a forward control channel may include such things as a cell""s identification, associated network identification, system timing information and other information needed to access the wireless system from a subscriber unit and to manage radio resources in the system. Reverse control channels are typically used for transmitting access requests from subscriber terminals. A channel used for this purpose may be referred to as random access channel (RACH).
An exemplary slot allocation, in particular, one utilized by wireless systems complying with the IS-136 standard, is illustrated in FIG. 3. For groups of three repeating slots on the uplink and downlink carrier frequency bands used by a base station, a xe2x80x9cslot pairxe2x80x9d on one pair of uplink and downlink carrier frequency bands is reserved for the provision of a forward Digital Control Channel (FDCCH), and a reverse DCCH (RDCCH), with other slots being assigned to Digital Traffic Channels (DTCs).
As illustrated in 4, the FDCCH has a plurality of xe2x80x9clogical channelsxe2x80x9d mapped thereon, including a multiplexed Broadcast Channel (BCCH) designed to convey information about system configuration and system access rules, and a multiplexed point-to-point short message service (SMS), paging and access response channel (SPACH). The BCCH is further divided into a Fast Broadcast Channel (F-BCCH) for conveying time-critical information such as system identification (ID) and registration information, an Extended Broadcast Channel (E-BCCH) for conveying less time critical information such as neighboring cell lists, and an SMS Broadcast Channel (S-BCCH). The SPACH comprises a short message service channel (SMSCH) for carrying messages, a paging channel (PCH) for conveying system pages, and an access response channel (ARCH) for providing system response to queries from subscriber units and other administration information. The RDCCH is used to provide a Random Access Channel (RACH), which is used by terminals to transmit requests to access the wireless system.
Wireless systems typically provide access on a xe2x80x9ccontention/reservationxe2x80x9d basis, controlled by information transmitted over FDCCHs and RDCCHs. As illustrated in FIG. 5, a Layer 1 (Physical Layer) message transmitted over the FDCCH typically is constructed from a Layer 3 message that is broken down into Layer 2 frames, a respective one of which is transmitted during a respective slot after convolutional coding and interleaving. Each Layer 1 FDCCH message includes coded Layer 3 data, along with a synchronization information (SYNC) field and a Coded Superframe Phase (CSFP) field that indicates the position of the FDCCH slot in a Superframe.
The FDCCH message also includes a Shared Channel Feedback (SCF) field that contains information about the reservation status of an associated RDCCH RACH. The reservation status information in SCF field includes a Busy/Reserved/Idle (BRI) field that indicates whether the corresponding RDCCH RACH is busy, reserved or idle. A Received/Not Received (R/N) field indicates whether a RACH burst was received on the corresponding RACH. A Coded Partial Echo (CPE) field may be used to identify a terminal for which a RACH burst has been successfully received.
The SCF may be utilized to control system access as follows. A mobile terminal seeking contention-based access xe2x80x9clistensxe2x80x9d to the FDCCH, examining the SCF BRI fields to find an available RDCCH RACH slot that is not reserved or currently in use by another terminal. Once a suitable slot is found, the mobile terminal transmits a RACH burst in the appropriate slot. Assuming that the base station receives the RACH burst and there is no contention from another terminal, the base station acknowledges receipt of the RACH burst in the SCF field of the next corresponding FDCCH slot. A mobile terminal can make a reservation-based access at the discretion of the base station, i.e., the base station transmits an FDCCH burst having an SCF field that indicates that a particular RACH slot is reserved for the mobile terminal, e.g., by setting the BRI field to xe2x80x9cIdlexe2x80x9d and including the seven least significant bits of the mobile terminal""s MSID in the CPE field.
Wireless communications systems are often subject to environmental effects that can render system access difficult. A wireless call which could be placed under system operating parameters that are designed to produce an acceptable level of communications quality under a set of nominal environmental conditions, may not be possible under xe2x80x9csub-nominalxe2x80x9d conditions of fading, shadowing by intervening objects such as hills, and attenuation by distance and by structures such as buildings.
High-penetration messaging and paging solutions have been proposed that allow a base station to transmit a short alphanumeric message to a terminal in a disadvantaged location, such as in a xe2x80x9cholexe2x80x9d between coverage areas or within a building or tunnel, using a high-penetration control channel. In response to the receipt of such a high-penetration short message, the mobile terminal can transmit a similar high-penetration acknowledgment, and later move to a less disadvantaged location and call back the calling party identified in the short message. Examples of high-penetration messaging services are described in U.S. patent application Ser. No. 09/193,261 (Rydbeck et al., filed Nov. 18, 1998) and U.S. patent application Ser. No. 09/195,790 (Rydbeck et al., filed Nov. 18, 1998), both of which are assigned to the assignee of the present invention. Although such HP-SMS services can provide valuable additional services, they generally provide only limited functionality because they generally do not provide the subscriber with full access to the wireless system.
In light of the foregoing, it is an object of the present invention to provide improved systems and methods for accessing a wireless communications system.
It is another object of the present invention to provide systems and methods for accessing a wireless communications system under disadvantaged radio propagation conditions.
These and other objects, features and advantages are provided according to the present invention by systems and methods in which a base station transmits a high-penetration channel reservation status indicator in a plurality of slots assigned to a high-penetration forward control channel responsive, for example, to receipt of a high-penetration access request transmitted from a terminal in a plurality of slots assigned to a high-penetration reverse control channel. The high-penetration channel reservation status indicator preferably comprises channel reservation status information that is coded according to an error correction code, more preferably a block code. For example, respective block code words representing channel reservation status information may be transmitted in respective Shared Channel Feedback (SCF) fields of a plurality of high-penetration forward Digital Control Channel (HP-FDCCH) slots. In this manner, access can be provided for terminals in disadvantaged locations while retaining message formats and protocols used for normal access. According to other aspects of the present invention, high-penetration hyperframe/superframe structures are used that group high-penetration subchannels such that an access request or a channel reservation indication is transmitted over a time period greater than the duration of a high-penetration superframe and less than the duration of a high-penetration hyperframe. These structures can gain the advantage of time diversity without incurring inordinately long message delays.
According to an aspect of the present invention, access is provided to a wireless communications system including at least one base station operative to communicate with one or more terminals over a forward control channel and a reverse control channel. A high-penetration channel reservation status is transmitted indicator from a base station in a plurality of slots assigned to a high-penetration forward control channel, such that the high-penetration channel reservation status indicator has a redundancy greater than that of a channel reservation status indicator transmitted over the forward control channel. Transmission of the high-penetration channel reservation status may be preceded by transmission of a high-penetration access request from a terminal in a plurality of slots assigned to a high-penetration reverse control channel, such that the high-penetration access request has a redundancy greater than that of an access request transmitted over the reverse control channel, and reception of the high-penetration access request at the base station. The high-penetration channel reservation status indicator may be transmitted responsive to receipt of the high-penetration access request.
According to another aspect of the present invention, channel reservation status information is coded according to an error correction code. A high-penetration channel reservation status indicator is transmitted by transmitting the error correction coded channel reservation status information. The error correction code may comprise a combination of a convolutional code and a block code.
According to other aspects of the present invention, high-penetration hyperframe/superframe structures are utilized. Respective successive groups of slots of a first physical channel are assigned to respective groups of high-penetration reverse control subchannels. Respective successive groups of slots of a second physical channel are assigned to respective groups of high-penetration forward control subchannels. A high-penetration access request is transmitted as a plurality of bursts in slots assigned to one of the high-penetration reverse control subchannels. A high-penetration channel reservation status indicator is transmitted as a plurality of burst in slots assigned to a corresponding one of the high-penetration forward control subchannels. Slots of a group of the groups of slots of the first physical channel may be assigned to high-penetration reverse subchannels in repeating sets of high-penetration reverse subchannel slots, and slots of a group of the groups of slots of the second physical channel may be assigned to high-penetration forward subchannels in repeating sets of high-penetration forward subchannel slots. A high-penetration reverse control channel hyperframe is defined, comprising a plurality of high-penetration reverse control channel superframes, a respective one of which includes a respective plurality of the repeating sets of high-penetration reverse subchannel slots. A high-penetration forward control channel hyperframe is defined, comprising a plurality of high-penetration forward control channel superframes, a respective one of which includes a respective plurality of the repeating sets of high-penetration forward subchannel slots, interleaved with slots reserved for synchronization bursts. A high-penetration access request is transmitted as a series of bursts within slots assigned to one of the high-penetration reverse control subchannels such that the access request is transmitted over a time period greater than the duration of one of the high-penetration reverse control channel superframes and less than the duration of the high-penetration reverse control channel hyperframe. A high-penetration channel reservation status indicator is transmitted as a series of bursts within slots assigned to one of the high-penetration forward control subchannels such that the channel reservation status indicator is transmitted within a time period greater than the duration of one of the high-penetration forward control channel superframes and less than the duration of the high-penetration forward control channel hyperframe.
In an embodiment according to the present invention, a base station is provided for communicating with terminals over a forward control channel and a reverse control channel. The base station includes a receiver operative to receive an access request in a slot assigned to the reverse control channel and to receive a high-penetration access request from a terminal in a plurality of slots assigned to a high-penetration reverse control channel, such that the high-penetration access request has a redundancy greater than that of an access request transmitted over the reverse control channel. The base station also includes a transmitter operative to transmit a channel reservation status indicator in a slot assigned to the forward control channel and to transmit a high-penetration channel reservation status indicator in a plurality of slots assigned to a high-penetration forward control channel, such that the high-penetration channel reservation status indicator has a redundancy greater than that of a channel reservation status indicator transmitted over the forward control channel.
In another embodiment according to the present invention, a terminal is operative to communicate with a base station over a forward control channel and a reverse control channel. The terminal includes a receiver operative to receive a channel reservation status indicator in a slot assigned to the forward control channel and to receive a high-penetration channel reservation status indicator in a plurality of slots assigned to a high-penetration forward control channel, such that the high-penetration channel reservation status indicator has a redundancy greater than that of a channel reservation status indicator received over the forward control channel. The terminal also includes a transmitter operative to transmit an access request in a slot assigned to the reverse control channel and to transmit a high-penetration access request in a plurality of slots assigned to a high-penetration reverse control channel, such that the high-penetration access request has a redundancy greater than that of an access request transmitted over the reverse control channel.